Biologically active polymers such as proteins, enzymes, and nucleic acids (DNA and RNA) have been delivered to the cells using amphipathic compounds that contain both hydrophobic and hydrophilic domains. Typically these amphipathic compounds are organized into vesicular structures such as liposomes, micellar, or inverse micellar structures. Liposomes can contain an aqueous volume that is entirely enclosed by a membrane composed of lipid molecules (usually phospholipids) [New 1990]. Positively-charged, neutral, and negatively-charged liposomes have been used to deliver nucleic acids to cells. For example, plasmid DNA expression in the liver has been achieved via liposomes delivered by tail vein or intraportal routes. Positively-charged micelles have also been used to package nucleic acids into complexes for the delivery of the nucleic acid to cells
Polymers have also been widely used for the delivery of biologically active polymers to cells. A number of drug delivery applications utilize polymer matrices as the drug carrier. Polymers have been used for the delivery of nucleic acids (polynucleotides, oligonucleotides, and RNA's) to cells for research and therapeutic purposes. This application has been termed transfection and gene therapy or anti-sense therapy, respectively. One of the several methods of nucleic acid delivery to the cells is the use of DNA-polycation complexes. It was shown that cationic proteins like histones and protamines or synthetic polymers like polylysine, polyarginine, polyornithine, DEAE dextran, polybrene, and polyethylenimine are effective intracellular delivery agents. Polycations are a very convenient linker for associating specific receptors or ligands with the nucleic acid-polycation complex, and as a result, nucleic acid-polycation complexes can be targeted to specific cell types. Polycations also protect nucleic acid in the complexes against nuclease degradation. This protection is important for both extracellular and intracellular preservation of nucleic acid.
The main mechanism of nucleic acid translocation to the intracellular space might be non-specific adsorptive endocytosis. Gene delivery using cationic polymers may be increased by preventing endosome acidification such as with NH4Cl or chloroquine. Some polymers, such as polyethylenimine and poly(propyl acrylic) acid, may also possess membrane disruptive or endosomalytic properties. Several reports have attributed the gene delivery properties of polyethylenimine to a buffering or proton sponge effect. Disruption of endosomes has also been reported as a result of linking to the polycation endosomal-disruptive agents such as fusion peptides or adenoviruses.
Polycations can also facilitate nucleic acid condensation. The volume which one DNA molecule occupies in a complex with polycations is dramatically lower than the volume of a free DNA molecule. The size of a DNA/polymer complex is probably critical for gene delivery in vivo. In terms of intravenous injection, DNA needs to cross the endothelial barrier and reach the parenchymal cells of interest. The largest endothelia fenestrae (holes in the endothelial barrier) occur in the liver and have an average diameter of 100 nm. The trans-epithelial pores in other organs are much smaller, for example, muscle endothelium can be described as a structure which has a large number of small pores with a radius of 4 nm, and a very low number of large pores with a radius of 20–30 nm. The size of the DNA complexes is also important for the cellular uptake process. After binding to the cells the DNA-polycation complex are likely taken up by endocytosis. Therefore, DNA complexes smaller than 100 nm are preferred.